With the gradual improvement of the requirement of capacity and flexibility of the optical communication system, the coherent optical communication technology becomes more and more important. In the coherent optical communication, the polarization multiplexing is usually adopted to simultaneously modulate the transmitted signals into two polarization states of the light, so as to multiply the transmission rate under the same bandwidth. At the receiving end, an optical coherent receiver simultaneously receives the signals under two polarization states and converts them into baseband digital signals. The signals under two polarization states are separated through a demultiplexing technology in the digital field, and subsequently, the signals under each polarization state are processed respectively. As can be seen from the above description, whether the signals under two polarization states are well separated has a great influence on the performance of the optical coherent receiver for polarization multiplexing.
In the optical communication system, an equalization and polarization demultiplexer is usually used. The equalization and polarization demultiplexer is implemented by an adaptive filter in general, while the coefficient of the adaptive filter may be adjusted by using constant modular algorithm, minimum mean squared error algorithm, etc. These algorithms all adjust the coefficient of the filter according to the channel state change by using the feedback structure, so that there is no mutual crosstalk between the signals under polarization states H and V.
Existing studies show that when the polarization state change in the channel is at a KHz magnitude, the adaptive filter can track the polarization state change at such magnitude. However, due to various reasons such as the channel nonlinear effect, the polarization state change in the channel may reach a magnitude of signal transmission rate (GHz). Since the current adaptive filter cannot track a polarization state change of such a high rate, crosstalk may occur between the output signals under Polarization states H and V due to residual polarized dispersion (see G. Charlet, et al., “Performance comparison of singly-polarized and polarization multiplexed at 10 Gbaud under nonlinear impairments”, OThu8, proceeding of OFC 2008).
For this reason, a method for compensating polarization crosstalk using a Polarization Crosstalk Canceller (PCC) has been proposed (e.g., a literature by Lei Li, OFC2010 paper OWE3). In this literature, the average length is predetermined. But the optimum average length is closely related to the actual condition of the communication system, and the predetermined average length sometimes cannot keep the system performance being in the optimum state.
To be noted, the above introduction to the technical background is just for the convenience of clearly and completely describing the technical solutions of the present invention, and facilitating an appreciation by a person skilled in the art. It shall not be considered that the above technical solutions are known to a person skilled in the art just because they are set forth in the Background section of the present invention.